1. Field of the Invention
The present invention relates to variable-frequency filters, and more particularly, to a variable-frequency band-elimination filter for changing a frequency band using a switching element.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 10-289342 discloses a communication system in which switching between a plurality of filters having pass bands corresponding to different frequency bands is performed using switches.
That is, a switch is disposed at the input ends of two signal paths passing through filters having different filter characteristics, another switch is disposed at the output ends of the two signal paths, and switching between the filter characteristics is performed by operating each of these switches.
Japanese Unexamined Patent Application Publication No. 7-321586 discloses a filter in which filters having different characteristics are individually disposed in a plurality of paths and a frequency characteristic is changed by performing switching between these paths using switches.
Japanese Unexamined Patent Application Publication No. 7-321586 discloses a variable frequency band filter. In the variable frequency band filter, a low-pass notch filter is obtained by connecting LC series resonance circuits in shunt with each other, and a pass band and an attenuation band are changed in accordance with a used transmission channel by changing capacitance values using diodes. The configuration of the variable frequency band filter disclosed in Japanese Unexamined Patent Application Publication No. 7-321586 will be described with reference to FIG. 1.
As illustrated in FIG. 1, in the variable frequency band filter disclosed in Japanese Unexamined Patent Application Publication No. 7-321586, a first series resonance circuit defined by a capacitor C1 and a coil L1 which are on the side of an input terminal 1 and a second series resonance circuit defined by a capacitor C2 and a coil L2 which are on the side of an output terminal 2 are connected to each other via a coupling coil L4 as a parallel arm. Coils L3 and L5 are disposed in parallel to the first and second series resonance circuits, respectively. A first tap is disposed at the coil L1 which functions as a band elimination filter having the resonance frequency of the first series resonance circuit as an attenuation pole, and a second tap is disposed at the coil L2 which functions as a band elimination filter having the resonance frequency of the second series resonance circuit as an attenuation pole. A switching diode D1 and a high-frequency bypass capacitor C3 are disposed between the first tap and the ground, and a switching diode D2 and a high-frequency bypass capacitor C4 are disposed between the second tap and the ground. In the variable frequency band filter having the above-described configuration, it is possible to establish a short circuit between each of the taps at the coils L1 and L2 and the ground by applying a control voltage to the anodes of the diodes D1 and D2 and to change the frequencies of the attenuation poles.
However, in the systems disclosed Japanese Unexamined Patent Application Publication No. 10-289342 and Japanese Unexamined Patent Application Publication No. 9-199996, it is necessary to provide a plurality of filters so as to perform switching between predetermined frequencies. As a result, the size of a transmission/receiving module is increased, and a system configuration is complicated.
The variable frequency band filter disclosed in Japanese Unexamined Patent Application Publication No. 7-321586 is used as a low-pass filter having a band elimination characteristic. The pass band of the variable frequency band filter is only on the lower side of the resonance frequencies of the LC series resonance circuits connected in shunt to the signal line. Since the combination of a diode and a capacitor is used to change a capacitance value, the parasitic inductance component of the diode is added to the capacitor when the diode is in an ON state. As a result, the filter characteristics of the variable frequency band filter are deteriorated.